The present invention relates to a method to reduce a DC-offset in at least one burst of a first digital signal by subtracting from samples of the at least one burst a correction value obtained by an averaging operation. It is also directed to a circuit arrangement to reduce a DC-offset in at least one burst of a first digital signal, the arrangement including substracting means, adapted to substract from samples of the at least one burst a correction value; averaging means, adapted to determine the correction value, and having an output coupled to an input of the subtracting means. It is further directed to a and burst mode receiver using such a circuit arrangement.
Such a method to reduce a DC-offset and related equipment are already known in the art, e.g. from the U.S. Pat No. 4,873,702, entitled xe2x80x9cMethod and Apparatus for DC Restoration in Digital Receiversxe2x80x9d.
Therein, a method is described for reducing the DC-offset in a digital signal by subtracting a correction value, the so-called offset voltage, from the samples of the digital signal. The offset voltage, as indicated in column 4 of the just cited U.S. Patent from line 15 to line 45, is obtained via an averaging operation. In U.S. Pat. No. 4,873,702 the averaging operation is performed on a preamble signal, i.e. a signal whose samples have predetermined values. A disadvantage of the known method is that transmission of such a preamble signal implies occupancy of bandwidth on the transmission medium between transmitter and receiver. This occupied bandwidth is no longer available for transmission of data. Furthermore, the use of a preamble signal or signal with a fixed predetermined contents for DC-offset purposes requires knowledge of the contents of this preamble signal at the receiver""s side. Indeed, the averaged preamble signal contains no information about the DC-offset picked up by a signal transmitted between transmitter and receiver or added to a signal by components in the receiver, for instance a mixer in a radio-receiver. The average of the preamble may differ from zero, even if no DC-offset was applied to the transmitted signal or may be zero even if there is a significant DC-offset. Another disadvantage of the known method is that the receiver has to be well-synchronised to the arrival of the preamble so that only samples of the preamble are averaged.
An object of the present invention is to provide a method and circuit arrangement to reduce a DC-offset such as the known one, but which have no influence on bandwidth occupancy on the transmission medium between transmitter and receiver, and which do not require knowledge of the contents of a signal received and averaged by the receiver.
According to the present invention, this object is achieved a method to reduce a DC-offset in at least one burst of a first digital signal by subtracting from samples of the at least one burst a correction value obtained by an averaging operation, wherein the correction value is calculated by averaging at least part of samples of a burst of the first digital signal, the samples not having a predetermined value.
The object is also achieved by a circuit arrangement to reduce a DC-offset in at least one burst of a first digital signal, the arrangement including: substracting means, adapted to substract from samples of the at least one burst a correction value; averaging means, adapted to determine the correction value, and having an output coupled to an input of the subtracting means, wherein the arrangement further includes: memory means to an input terminal of which a burst of the first digital signal is applied, and an output terminal of which is coupled to an input terminal of the averaging means, the memory means being adapted to temporarily store the burst and to apply at least part of the samples thereof which have no predetermined value to the averaging means.
This object is still further achieved by a burst-mode receiver used to receive bursts of an analogue radio signal, the burst-mode receiver including: an antenna, adapted to convert the bursts of the analogue radio signal into bursts of an analogue electrical signal; a radio frequency component, coupled to the antenna and adapted to transform the bursts of an analogue electrical signal into bursts of a digital baseband signal; an offset reducing arrangement, coupled to the radio frequency component and adapted to reduce a DC-offset in at least one burst of the digital baseband signal; and a baseband component, coupled to the offset reducing arrangement and adapted to digitally process the bursts of the digital baseband signal; the offset reducing arrangement including: subtracting means, adapted to subtract from samples of the at least one burst of the digital baseband signal a correction value; and averaging means, adapted to determine the correction value, and having an output coupled to an input of the subtracting means, wherein the offset reducing arrangement further includes: memory means to an input terminal of which a burst of the digital baseband signal is applied, and an output terminal of which is coupled to an input terminal of the averaging means, the memory means being adapted to temporarily store the burst and to apply at least part of the samples thereof which have no predetermined value to the averaging means.
In this way, by averaging samples of a data burst which have no predetermined value, a correction value is obtained without transmission of a predetermined sequence of samples or preamble. Precise synchronisation between the averager and the arrival of the data bursts further is not important since any arbitrary sequence of samples of the data burst allows the receiver to calculate a correction value. This correction value is a good DC-offset estimator in any system wherein the average of the transmitted signals is zero. Its quality as DC-offset estimator is less good if the transmitted signals do not have this property.
It is noticed that the DC-offset reducing method from U.S. Pat. No. 4,873,702 includes an additional phase, the so called second step, wherein the offset correction value is updated upon information deduced from the data sent between transmitter and receiver. In this second step, no subsequent data samples are averaged to determine how the offset correction value has to be modified. The second step hence cannot be used to determine an initial offset correction value and can therefore not be regarded as an autonomous offset reducing method. The second step described in U.S. Pat. No. 4,873,702 requires a preceding first step wherein an initial offset reducing correction value is determined. The present invention provides such a method which is advantageous over the known first step method, as explained above.
It is also remarked that a method and circuit, similar to the one known from U.S. Pat. No. 4,873,702 is further known from the European Patent Application EP 92122006.7, entitled xe2x80x98Method and Circuit Arrangement for Offset Correction in a TDMA Radio Receiverxe2x80x99. The TDMA radio receiver described therein is provided with a circuit which determines a DC-offset correction value by averaging the middle 64 bits of a 142 fixed bits long time interval. Although this circuit doesn""t average preamble samples, a person skilled in the art will recognise that the method disclosed in EP 92122006.7 suffers the same drawbacks as that of U.S. Pat. No. 4,873,702.